The invention relates to copolymerizable photoinitiators useful for the photopolymerization of ethylenically unsaturated compounds and also to systems containing such compounds.
Photochemically induced polymerization reactions have attained great importance in industry, particularly in the rapid hardening of thin films, as, for example, in the hardening of paint or resin coatings on paper, metal and plastics or in the drying of printing inks, since these processes are distinguished from conventional methods for printing and coating articles by raw material and energy savings and reduced environmental pollution. However, the preparation of polymeric materials by polymerizing appropriate unsaturated monomeric starting materials is frequently effected photochemically, for which customary processes such as solution and emulsion polymerizations can be used.
Since with the aforementioned reactions it is generally the case that none of the reactants is capable of absorbing photochemically active radiation to an adequate degree, it is necessary to add so-called photoinitiators which are either capable of absorbing incident high-energy radiation, usually UV light, and of forming active starter radicals which in turn initiate photopolymerization, or are capable of transferring the absorbed energy for free radical formation to one of the polymerizable reactants. The initiators do not normally participate in the actual polymerization reaction.
The initiators used hitherto for photopolymerizing unsaturated compounds have been in the main benzophenone derivatives, benzoin ethers, benzil ketals, dibenzosuberone derivatives, anthraquinones, xanthenones, thioxanthones, .alpha.-halogenactophenone derivatives, dialkoxyacetophenones and hydroxyalkyl phenones.
Essential criteria for the selection of such initiators are, e.g., the nature of the reactions to be carried out, the ratio of the absorption spectrum of the initiator to the spectral energy distribution of the available source of irradiation, the activity of the initiator, the solubility of the initiator in the reaction mixture, the dark storage life of the reaction system to which the initiator has been added, and the action on the end products by residues remaining therein of the initiator and/or of the products formed therefrom in the course of the photochemical reaction.
As is known, however, the technical feasibility of many of the substances mentioned is restricted, in some instances severely, by a number of disadvantages. These particularly include insufficient reactivity to initiate the photopolymerization of ethylenically unsaturated compounds. In addition to molecule-specific reactivity, the crucial factor here in many cases is the solubility, or the ideally uniform incorporability of the photoinitiators in the photopolymerizable systems.
However, owing to their favorable properties, the hydroxyalklphenones of German Offenlegungsschrift No. 2,722,264, corresponding to U.S. Pat. Nos. 4,347,111 and 4,477,681, have proved to be particularly advantageous. This specific class of substances is present at the typical application temperature range in the liquid state; this results in an excellent solubility or a homogeneous incorporability in conventional photopolymerizable systems. Furthermore, these photoinitiators are of an above average activity and at the same time the systems to which they are added have a remarkably favorable dark storage life. Nonetheless, hydroxyalkylphenone photoinitiators are still improvable.
Since these compounds, like the other conventional photoinitiators, do not participate in the actual photopolymerization, their excess, unreacted residues and their degradation products formed during the photochemical reaction remain in the finished product. Depending on the precise nature and amount of the product this can lead to more or less pronounced effects on the properties of the product. In the case of photopolymerized paint coatings, the main field of use for photoinitiators, such residues can, for examples, affect the obtainable final hardness of the layer. Also, it is possible for undesirable color changes, for example, yellowing, to occur, frequently only after a prolonged time period. Initiator residues and/or degradation products, owing to their more or less pronounced volatility, can become noticeable through unpleasant odor; their diffusion out of the coating and into surrounding media can create problems, for example where packaging materials with photopolymerized coatings, such as, for example, tins and tubes, are provided for foodstuffs. Particularly in this field of use the question of utility is crucially determined by the possible or known toxicity of the photoinitiators and their degradation products.
The prior art discloses certain photoinitiators which have unsaturated substituents, as well as other classes of substituents. They are predominantly benzoin and benzil derivatives. However, apart from the compound .alpha.-allylbenzoin, which in its initiator action approaches the hydroxyalkylphenones, such unsaturated photoinitiators have not become established, since they do not offer any significant advantages with respect to content of unreacted residues and degradation products in the polymer product, and in other aspects are inferior to the hydroxyalkylphenones.